The invention relates generally to the measurements of magnetic fields and more specifically concerns the measurements of very low magnetic fields.
Magnetic measurements are expected to provide useful information in a number of areas such as mineral prospecting surveys, planetary field measurements, and archaeological investigations, as well as laboratory studies. There ae several types of magnetometers currently available, principal among which are: (1) Pivoted needle instruments which include the Swedish mine compass, the Hotchkiss superdip, and the Thalen-Tuberg magnetometers. The accuracy of these instruments seldom exceeds .+-.100 nT (1 nT=1 gamma=10.sup.-5 gauss), and they are not used much in current high-resolution applications. (2) The Schmidt and compensation-type variometers are precision magnetometers having accuracies better than .+-.5 nT, although their field accuracies are often in the range of .+-.(25 to 50) nT. (3) Fluxgate instruments can directly measure the vector components of the magnetic field. One property that makes fluxgate magnetometers suitable for ground as well as space use is their wide measurement range and low noise level. A sensitivity of 1 nT is routinely possible with airborne fluxgate meters, although sensititivities approaching 0.01 nT in the 0 to 10 Hz bandwidth and zero level stability of the order of .+-.0.1 nT have been reported for low field fluxgate meters designed for deep space missions. The NASA Goddard fluxgate magnetometers onboard Voyager 1 have been reported to have a preliminary accuracy of .+-.0.2 nT .+-.0.1 percent of full scale. (4) Proton precession magnetometers measure precession of spin-aligned protons around the test field after a strong magnetic field is removed from a sample of water. The spin-aligned protons precess around the test field H at a frequency given by g.sub.p H, where g.sub.p is the proton gyromagnetic ratio. These magnetometers offer a sensitivity of 1 nT at a 1-sec sampling rate. (5) Optically pumped magnetometers, such as helium and alkali vapor magnetometers, are among the most sensitive types of magnetometers and can measure fields as low as 0.01 nT. (6) A superconducting quantum interference device (SQUID) is the most sensitive magnetometer today. A SQUID magnetometer is essentially a superconducting flux transformer tightly coupled to a SQUID and can routinely achieve resolutions of better than 10.sup.-5 nT/(Hz).sup.1/2. Frequently, SQUID magnetic field gradiometers are more practical than the absolute SQUID magnetometers. The gradiometers call for the use of a flux transformer with two pickup coils of equal area arranged to induce zero supercurrent in a uniform magnetic field and have reported sensitivities of less than 5.times.10.sup.-7 nT/cm/(Hz).sup.1/2.
To date, the fluxgate magnetometers have had the highest use in geomagnetic as well as interplanetary field mapping studies. These instruments have a routine sensitivity of .perspectiveto.1 nT, although some designed for planetary and interplanetary field measurements have been reported to have much better sensitivities. For example, the low field magnetometers on the Voyager 1 and 2 missions have an estimated absolute measurement accuracy of 0.09 nT, although changes in fields smaller than 0.09 nT can be detected since their observation is limited only by the sensor RMS noise (0.006 nT) and the quantization step size (0.004 nT for the sensitive range). Thus, it would appear reasonable to consider the development of an instrument that has a routine sensitivity of at least two orders of magnitude better (i.e., .ltoreq.0.01 nT). Among the existing instruments, there are two types that meet this sensitivity requirement, namely, the optically pumped magnetometers and the SQUID magnetometers. However, the SQUID magnetometer needs liquid helium for its operation. Hence, despite its extremely high stability and sensitivity, it seems destined to be used in the laboratory for special purposes only. The optical absorption magnetometers, which can measure the total field as well as the vertical gradients, have a routine sensitivity of &lt;0.1 nT and are suitable for diverse laboratory and field (including space) applications. The rubidium magnetometer has commonly been used with low-altitude, Earth-orbiting satellites to obtain geomagnetic maps above the ionosphere. The Vector helium magnetometer has been flown on planetary missions to Mars, Venus, and Jupiter.
It is therefore the primary object of this invention to provide a magnetometer that will make very low magnetic field measurement.
Another object of this invention is to provide a magnetometer that is extremely sensitive.
A further object of this invention is to provide a magnetometer that has a fast response time.
In accomplishing these objects the prior art electron beam magnetometer is utilized. For this type of magnetometer the deflection of the electron beam in the test magnetic field is dependent on the electron velocity. It is therefore desirable that the electrons in the electron beam have low energy or velocity.
Still another object of this invention is to provide a low energy electron magnetometer (LEEM).
It is possible to use lower electron energy without adversely affecting the sensitivity of the LEEM if the initial energy straggle and nonuniform angular distribution of the electrons in thermionic emission can be eliminated. The energy straggle or electron energy spread will produce a beam-spot distortion in the magnetic field which will adversely affect magnetometer sensitivity and the nonuniform angular distribution of the electrons causes the beam spot to be nonuniform which affects sensitivity.
A still further object of this invention is to provide a LEEM in which the electrons in the electron beam are near monoenergetic.
Yet another object of this invention is to provide a LEEM in which the electrons in the electron beam have random angular distribution.
Yet still another object of this invention is to increase the deflection of the electron beam in an electron beam magnetometer without increasing the length of the magnetometer.
Yet a still further object of this invention is to increase the uniformity of the electron beam in an electron beam magnetometer.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.